1. Field of the Invention
The present invention generally relates to an image recording apparatus, and more particularly to an image recording apparatus capable of adjusting the sensitivity to suit the ambient temperature change.
2. Description of Prior Art
Observation or documentation in ecology is generally proceeded by an image recording apparatus and using a pyroelectric infrared sensor (PIR) as a detector. Once an animal enters the predetermined zone, the PIR measures that the difference of the ambient temperature change caused by the animal which exceeds a specific quantity, the image recording apparatus can be triggered to activate and start to record images or videos. According to prior arts, a comparing circuit may be utilized to compare the sensing signal of the PIR with a predetermined threshold. If the sensing signal is higher than a high threshold voltage or lower than a low threshold voltage, comparator in the comparing circuit will output a trigger signal. The image recording apparatus receives the trigger signal and then will enter into the work mode to proceed the image capturing operation.
However, the observation or documentation in ecology is generally proceeded in the field or in the jungle where the ambient temperature change is enormous. Comparing the sensing signal with the fixed high and low threshold voltages can result in that the comparing circuit becomes oversensitive or in the opposite way. In case that the body temperature of the animal is at 36 degrees Celsius and the ambient temperature rises from 25 degrees Celsius to 40 degrees Celsius, the temperature difference is 11 degrees Celsius when the ambient temperature is 25 degrees Celsius. Therefore, the animal enters the predetermined zone can be confirmed for sure. But when the ambient temperature rises to 40 degrees Celsius, the temperature difference becomes only 4 degrees Celsius. Even the animal entered the predetermined zone, the detector is now insensitive and the image recording apparatus may not be activated and the chance of observation or documentation is lost.
On the contrary, in case that the body temperature of the organism is at 25 degrees Celsius which is not the recording target object and the ambient temperature drops from 25 degrees Celsius to 15 degrees Celsius, there is no temperature difference when the ambient temperature is 25 degrees Celsius. Therefore, the image recording apparatus will not be activated. But when the ambient temperature drops to 15 degrees Celsius, the temperature difference becomes 10 degrees Celsius. The detector is now oversensitive and the image recording apparatus will be activated and result in a wrong shoot or a maloperation.
For solving the maloperation problem caused by the ambient temperature change as aforementioned, a thermistor is connected across the input terminal of threshold signal of the comparing circuit and the output terminal of the comparing circuit. By sensing the ambient temperature change with the thermistor to result in the resistance change, the gain of the comparator of the comparing circuit is adjusted by a voltage signal outputted from the thermistor.
Please refer to FIG. 1-A and FIG. 1-B, which depict oscillograms of output signals when the ambient temperatures are higher and lower according to the prior arts. The dotted lines represent the original output signals Voriginal of the comparing circuit without the thermistor. The full lines represent the real output signals Vreal of the comparing circuit with adding the thermistor to couple therewith. The condition is that the fixed high and low threshold voltages are used to compare with the sensing signal. As shown in FIG. 1-A when the ambient temperature is higher, the thermistor senses a higher temperature and generates the resistance change. The gain of the comparator is uplifted to make the real output signal Vreal higher than the original output signal Voriginal and therefore, to raise the sensitivity of the image recording apparatus. When the ambient temperature rises, the temperature difference becomes smaller and therefore, the original output signal Voriginal is too small to exceed the predetermined threshold voltages. However, uplifting the gain of the comparator with the thermistor to raise the sensitivity, the insensitivity issue can be solved when the animal enters the predetermined zone but the image recording operation is not proceeded. The correct image recording operation can be carried out.
On the contrary and as shown in FIG. 1-B, when the ambient temperature is lowered, the thermistor senses a lower temperature and generates the resistance change. The gain of the comparator is diminished to make the real output signal Vreal lower than the original output signal Voriginal and therefore, to reduce the sensitivity of the image recording apparatus. When the ambient temperature drops, the temperature difference becomes larger and therefore, the original output signal Voriginal is too large to exceed the predetermined threshold voltages. However, diminishing the gain of the comparator with the thermistor to reduce the sensitivity, the oversensitivity issue can be solved when the organism, which is not the recording target object, enters the predetermined zone and the image recording operation is proceeded. The image recording maloperation can be avoided.
According to the aforesaid prior arts, adjusting the gain of the comparator based on the ambient temperature change can provide the function of sensitivity adjustment. However, such gain uplifting or diminishing method of the comparator not only amplifies the output signal but also amplifies the noise at the same time. Consequently, the comparison result also contains more uncertainty.